The present invention generally relates to a communication adaptor for reproducing cells received from an ATM (Asynchronous Transfer Mode) network to original information in a multiplex communications system using an ATM communication method. More particularly, the present invention relates to a regulation method and apparatus for ATM cell delay variation.
A multiplex communications system using the ATM communication method performs communications in the following manner. Information to be transferred is transformed into a fixed length short packet called a cell on the transmission side and then transmitted to an ATM network. A switch in the ATM network selects a transmission path with reference to a header in the cell, and a cell received from the ATM network is reproduced to original information on the reception side.
In the ATM communication system as mentioned above, while the transmission side transmits cells at regular intervals, the cells may collide on the network when a switch transfers them to a next transmission path so that the cells may reach the reception side at irregular intervals, thus causing a delay variation in the ATM cells. This phenomenon is called xe2x80x9cATM cell delay variationxe2x80x9d in this disclosure. This is a phenomenon inherent to the ATM transmission method and is never found in an STM (Synchronous Transfer Mode) transmission method.
For regulating the ATM cell delay variation, the following method may be employed. Instead of reproducing transferred cells to original information at the same time they reach the reception side, the transferred cells are temporarily stored in a buffer memory, and after the lapse of a predetermined time from the reception of the first cell in a cell stream generated in a burst manner, the cells are read from the buffer memory at the same intervals as they have been transferred from the transmission side to reproduce original information therefrom.
FIG. 12 is a block diagram illustrating the configuration of an ATM cell delay variation regulation unit according to the conventional method using a buffer memory. Referring specifically to FIG. 12, the ATM cell delay variation regulation unit includes a variation regulating buffer 11, a variation waiting timer 12, and a read-out timer 13.
When the first cell in a burst is transferred from a VPI (Virtual Path Identifier)/VCI (Virtual Channel Identifier) demultiplexing unit of the ATM network to the delay variation regulation unit, the variation waiting timer 12 is started, and the first and subsequent cells are stored in the variation regulating buffer 11. After a predetermined time has elapsed, the variation waiting timer 12 starts the read-out timer 13. The cells stored in the variation regulating buffer 11 are read out at the same intervals as cell transmitting intervals on the transmission side by the read-out timer 13 and transferred to a cell reproduction unit.
FIG. 13 is a timing chart representing the timing of cells associated with a conventional delay variation regulation method. Referring specifically to FIG. 13, cells are delivered from the transmission side at delivery intervals Tb (a). The cells reach the cell delay variation regulation unit with a varying delay amounts Td1-Td5 which are respectively the sum of a transmission delay Tc in the ATM network and a delay in the switch in the ATM network Tj1-Tj5 (b). Among the transmission delay Tc and the delays Tj1-Tj5 forming the delay amounts Td1-Td5, the transmission delay Tc is a fixed amount, whereas the delays Tj1-Tj5 in the switch are varying amounts (c). Thus, the read-out timer 13 reads the cells stored in the variation regulating buffer 11 at intervals equal to the delivery intervals Tb after a waiting time Tw from the time the first cell in a burst reaches and is stored in the variation regulating buffer 11 in the delay variation regulation unit (d).
For the waiting time Tw which extends from the time the first cell in a burst is received to the time it is read from the buffer 11, it is thought as a common tendency to employ a maximum delay time of the cell. On the other hand, JP-A-5-83289, laid open on Apr. 2, 1993, has proposed reducing the waiting time by employing as the waiting time a value calculated by subtracting a time required to reproduce original information from a cell from the maximum delay time of the cell.
The above-mentioned cell delay regulation method fixedly uses a value based on the maximum delay time of the cell as the waiting time which extends from the time the first cell is received in the buffer to the time it is read therefrom irrespective of a type of information.
However, when the maximum delay time of the cell or a time proximal to this is used as the delay variation waiting time, communications of signals sensitive to delays such as sound, moving images, and so on suffer from problems in terms of the service quality, i.e., delayed or tedious sound, deteriorated image quality, and so on.
It is an object of the present invention to provide a regulation apparatus for ATM cell delay variation for inserting a waiting time between the reception of cells into a buffer and the read-out of the same for regulating a delay variation, which is capable of variably setting the waiting time corresponding to a communication type to achieve an appropriate communication scheme for each communication type.
FIG. 1 is a block diagram illustrating the configuration of a cell delay variation regulation unit according to the present invention. The cell delay variation regulation unit comprises, in addition to the configuration of conventional delay variation regulation unit including a variation regulating buffer 31 for temporarily storing cells transferred from a VPI/VCI demultiplexing unit, a variation waiting timer 32 for controlling a waiting time which extends from the time the first cell in a burst is received to the time read-out of cells is started from the variation regulating buffer xe2x80x9c31xe2x80x9d, and a read-out timer 33 for controlling intervals at which cells are read from the variation regulating buffer xe2x80x9c31xe2x80x9d. The cell delay variation regulation unit further comprises a latest cell preservation memory 34 for storing the latest cell transferred from the VPI/VCI demultiplexing unit, and a selector for selecting which of the variation regulating buffer 31 and the latest cell preservation memory 34 a cell is read from to be transferred to a cell reproduction unit.
Also, a communication type discrimination unit 36 arranged internal or external to the cell delay variation regulation unit is provided with an information kind discrimination function for discriminating a communication type in a call set-up phase upon starting communications and for setting to the variation waiting timer 32 the waiting time which extends from the time the first cell in a burst is received to the time read-out is started from the variation regulating buffer 31 corresponding to the discriminated communication type (from steps S101 to S108 in FIG. 8).
Further, the cell delay variation regulation unit comprises a dummy cell inserting unit for inserting the last reaching cell as a dummy cell when the variation waiting time fails to regulate a cell delay time in a communication of sound or moving images (provided by the latest cell preservation memory 34 and the selector 35).
According to the present invention, the communication type discrimination function discriminates, in a call set-up phase upon starting a communication, whether the communication to be performed is associated with sound, moving images or the like which are largely affected by delayed cells, with a file transfer or the like which is largely affected by lost cells but not affected so much by delayed cells, or with remote terminal processing or the like which is largely affected both by delayed cells and by lost cells. The communication type discrimination function also calculates a variation waiting time in accordance with the communication type based on the result of the discrimination, and sets the calculated variation waiting time to the variation waiting timer 32, so that an appropriate communication quality can be provided for each communication type.
Also, since the dummy cell inserting unit can insert the last reaching cell as a dummy cell, when the variation waiting time fails to regulate a cell delay time in a communication of sound, moving images, or the like, to prevent interrupted sound or moving image information, deterioration of the quality can be minimized.